1. Field of the Invention
The present invention relates to unloading a head from a disk in a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
Disk drives typically employ a servo to maintain the heads on the centers of the tracks. Deviations from the track center line may create errors in the reading and/or writing of data. The servo utilizes servo bits written into servo fields of the disk. The servo bits are used to produce a position error signal (“PES”) that is processed to center the head on the disk track. Each track also includes cylinder information that can be read to determine the track location of the heads.
When a drive is not in use the heads are typically moved off of the disks. For example, the heads can be moved onto a mechanical ramp. As the heads move across the disks the cylinder information can be read and used to determine head location and speed. When the heads move off of the disks there is no longer track information. At this point, the back emf of the voice coil motor can be determined to calculate head speed.
The voltage of a voice coil can be described with the following equation:v=Ldi/dt+Ri+Bemf 
where;
L=The inductance of the voice coil.
di/dt=The time rate of change of the drive current.
i=The drive current.
R=The electrical resistance of the coil and wires.
Bemf=The back emf of the voice coil motor.
During an unload routine the drive current is typically a DC current, making the inductance component zero. The back emf can be determined by subtracting the DC component Ri from the voice coil voltage. The resistance value may vary because of heat and other factors. Consequently, a calibration routine must be performed during each unload routine to determine the value R before calculating the back emf. Such a routine increases the time required to unload the head.